1. Field oF the Invention
The present invention relates to a power supply control circuit for a power supply having constant current and constant voltage modes, and more particularly, to a power supply control circuit having a feedback network which renders the constant current and constant voltage control loop transfer functions simultaneously insensitive to the impedance of the power supply's load while still sharing a common output stage.
2. Description of the Prior Art
Power supplies having constant voltage (CV) and constant current (CC) modes are well known. However, prior art CV/CC power supplies generally have very broad dependencies between the power output stage and the control loops for the constant current and the constant voltage modes, and these dependencies greatly limit the performance of prior art power supplies. In other words, the design of prior art CV/CC power supplies invariably requires a balance of performance trade-offs associated with the output stage and the control loops of the power supply. Because of the costs associated with each of these circuit components, a good design has previously been one which trades off the benefits of each element with as little impact on cost and performance as possible. Unfortunately, these cost and performance trade-offs significantly limit how well the CV/CC power supply can operate in constant voltage or constant current mode.
An example of a prior art CV/CC power supply having a constant current preferred output stage is shown in FIG. 1. The CV/CC power supply of FIG. 1 generally includes a constant current control loop comprising elements 102-110 and a constant voltage control loop comprising elements 112-120. Both control loops include the output stage 108. During constant current operation of the circuit of FIG. 1, a constant current programming source supplies adder 102 with a predetermined constant current level at which a load connected to the output stage 108 is to be driven. The output of the adder 102 is amplified at constant current error amplifier 104 and then passed through a constant current gate diode 106 to drive the output stage 108. The output current detected at the output stage 108 is then fed back through current monitoring amplifier 110 to a negative input of adder 102 to form a negative feedback control loop. This control loop enables the current output of the output stage 108 to be maintained at the predetermined constant current level. Similarly, during constant voltage operation of the circuit of FIG. 1, a constant voltage programming source supplies adder 112 with a predetermined constant voltage level at which a load connected to the output stage 108 is to be driven. The output of the adder 112 is amplified at constant voltage error amplifier 114 and then passed through a constant voltage gate diode 116 to drive the output stage 108. The resulting output current flows through a load impedance 118 of the output circuitry, and the resulting output voltage is measured by voltage monitoring amplifier 120. The measured voltage is then fed back to a negative input of adder 112 to form a negative feedback control loop which enables the voltage across the load impedance 118 to be maintained at the predetermined constant voltage level.
Typically, CV/CC power supplies of the type shown in FIG. 1 have an output stage which favors either constant voltage operation or constant current operation. This ability to favor one mode of operation over another is defined by the output stage's ability to present at its output a voltage or current value fairly independent of the load impedance connected to the output stage while the input to the output stage is held constant. Hence, an output stage is classified as constant voltage preferred if when driven open loop from its input it exhibits the characteristics of a voltage source over that of a current source. Similarly, an output stage that when driven open loop from its input exhibits the characteristics of a current source is thought of as constant current preferred. The nature of this open loop transfer function of the output stage greatly influences the level of performance achievable by the constant current and constant voltage control loops in that output stage that are voltage preferred tend to yield excellent constant voltage performance but only moderate constant current performance due to the effects of the load impedance on the output of the output stage, while the opposite is true for output stages that are constant current preferred. However, these trade-offs between the two modes of operation of prior art power supplies are undesirable in that substantially ideal performance in both modes is inherently unattainable due to the adverse effects of the load impedance.
Since prior art CV/CC power supplies of the type shown in FIG. 1 generally trade-off the performance requirements by offering excellent performance in one mode but less than achievable performance in its other mode, application problems result because such power supplies are nevertheless expected to supply power both under constant voltage and constant current conditions and under a wide range of load conditions during operation. As a result, less than achievable performance of the power supply often results. Two basic approaches to the above problems have been used in prior art CV/CC power supplies. One approach has been to employ a constant voltage preferred output stage and to handle the constant current problems to the best extent possible. The other approach has been to start with a constant current preferred output stage and then to manage the constant voltage problems to the best extent possible. However, both of these prior art approaches have obvious limitations.
Because of the majority of applications requiring constant voltage operation, power supplies employing a constant voltage preferred output stage are commonly used in the prior art. These power supplies provide excellent constant voltage performance although the constant current loop can only be compensated to the extent possible to obtain limited performance in three major performance areas. These three areas are the ability to drive inductive loads, constant current recovery dynamics, and constant current noise performance. Unfortunately, when used with simple and inexpensive compensation schemes, these requirements tend to pull the design of the constant current control loop in two different directions. In prior art designs where the ability to drive highly inductive loads is pursued, the constant current control loop will tend to be compensated in a conservative fashion with very little bandwidth. This permits the constant current control loop to be more stable for inductive loads but also tends to yield a slow dynamic response in the time domain since when the power supply crosses over from constant voltage mode to constant current mode, the constant current loop which has been previously saturated must recover and slew back into regulation. A more sluggish compensation strategy for inductive loading will cause the constant current loop to recover slowly, during which time the output current of the power supply is unregulated and thus can damage sensitive loads by exceeding the constant current limit setting for a significant period of time.
Another problem with the constant voltage preferred approach is that high constant current output noise results, particularly excessive constant current RMS noise. The constant current RMS noise is also a result of the sluggish constant current loop compensation for inductive loading reasons. The constant current control loop thus tends to have less loop gain at nearly all frequencies and therefore makes it less capable of rejecting noise injected into the control loop from external noise sources. In addition, since the load impedance plays a significant role in the overall constant current loop gain, constant current performance can depend heavily on the actual load being driven. It is thus more difficult to specify constant current performance tightly without having to apply restricted load conditions. The constant current control loop thus has been dependent on the impedance of the load connected to the power supply.
As a result, previously it has been common practice to take a power supply employing a constant voltage preferred output stage that drives capacitive loads well and to heavily compensate its constant current loop in order to be able to drive highly inductive loads. However, although good results have been obtained in both modes for driving reactive loads, these results have been at the great expense of sluggish response when the supply is expected to rapidly cross-over from constant voltage mode to constant current mode under a load transition. Accordingly, it has taken a long period of time to get into constant current mode and/or a very large current overshoot has occurred causing possible damage to the load. As a result, prior art power supplies employing voltage preferred output stages typically have poorer inductive loading capabilities and have not enabled the full benefits of constant current mode operation to be achieved.
On the other hand, prior art power supplies which employ constant current preferred output stages typically drive inductive loads well inherently but may have a very large capacitance on their output. In particular, in prior art power supplies that employ constant current preferred output stages, the basic problem has been to deal with the variability of the load impedance presented to the output stage. In such prior art power supplies, there is a voltage gain from the input of the output stage to the output voltage of the power supply, and this voltage gain is directly dependent on the impedance of the load connected to the supply. A prior art proposal to eliminate the influence of the load impedance is to place a very low impedance, such as a large electrolytic capacitor, internal to the power supply but in parallel with the output terminals of the supply. This common technique stabilizes the output impedance for all loads where the load impedance is higher than that of the internal impedance. However, once such a capacitor has been chosen for the power supply design, it must be compensated for in the constant voltage control loop design. Thus, although this technique may solve the reactive loading problems, it forces the power supply to be slow with respect to up and down programming speed caused by the need to charge and discharge the large output capacitance.
As just noted, this approach has problems in that the output capacitor must be charged and discharged repeatedly in applications that require the output voltage of the supply to move between different values. The speed at which the output voltage can move depends on the size of the output capacitor and tends to make these power supplies slower than those with less output capacitance. Another drawback with this approach is that since the output capacitor is present all the time, it effectively lowers the output impedance when the power supply is in the constant current mode, which is less ideal. Moreover, the output capacitor itself is not an inexpensive or small component and adds significant cost to the power supply. Also, since there is non-negligible variability in the electrical parameters of the capacitor with respect to manufacturing tolerances, age and temperature, such variations must be taken into account in the worst case design of the control loop. The final worst case design will typically have degraded performance compared to a design that could have been less sensitive to this variability.
Furthermore, a large output capacitor has been a problem for some applications in the prior art due to its energy storage nature, for the larger the output capacitor, the more energy it stores. As a result, when a sudden load change occurs, all of the energy stored in the output capacitor can be dissipated in the load so as to cause damage, which is, of course, undesirable. Thus, the existence of the large capacitance has also led not only to increased cost but also to performance problems.
Accordingly, prior art power supplies have been unable to simultaneously meet a complete set of performance requirements for the constant voltage and constant current modes. Moreover, in accordance with the compensation strategies heretofore used, it has been inherently impossible to meet a complete set of performance requirements for both modes so that good performance may be achieved in the many possible combinations of subsets of the performance factors due to the interrelationship of the performance factors in both modes. A long-felt need in the art thus exists for a CV/CC power supply control circuit which enables the performance requirements in each mode to be met without the performance trade-offs which have been a problem in the prior art. The present invention has been designed to meet this need.